I have an Obsidian 9 myself, and did a lot of research on it. I settled on a Strike Industries Strike rail. It has an I.D. of 1.5" and can accommodate a can tucked inside. I have a 4" barrel with a 7" rail. Make sure you get the tri-lug adaptor for both the barrel and can. I use the Rugged adaptor for the can and a Hansohn Brothers for the barrel.
Other rail choices: Samson Evo, Midwest Industries, Seekins MCSR, Griffen Armament, and the DD MFR XL.
Edit: inleadwetrust also makes a tri lug flash can that will fit under a rail for when not using the suppressor.
I already have a Rugged 3-lug. Indeed, I have a few, but they’re all 1/2x28 of course. I assumed the barrel in PSA build kit I have was 1/2x28, but apparently it’s 1/2x36 for some reason, so I’ll need an adapter.